Airtightness is an important requirement for any energy-efficient building, **however, it is not the most important one** (as sometimes suggested in popular publications - the most important requirement is good thermal insulation). Example: Passing the [[planning:​airtight_construction:​general_principles:​blower_door_test|blower door test]] is a necessary prerequisite for the functioning passive building; yet it is not sufficient. Example: a conventional aircraft shell is airtight (much more airtight than a Passive House needs to be), but it is not insulated to Passive House quality In aircrafts there is abundant heat available for cabin heating.

Airtightness is an important requirement for any energy-efficient building, **however, it is not the most important one** (as sometimes suggested in popular publications - the most important requirement is good thermal insulation). Example: Passing the [[planning:​airtight_construction:​general_principles:​blower_door_test|blower door test]] is a necessary prerequisite for the functioning passive building; yet it is not sufficient. Example: a conventional aircraft shell is airtight (much more airtight than a Passive House needs to be), but it is not insulated to Passive House quality In aircrafts there is abundant heat available for cabin heating.